I am not sure of the taste difference, but I know demerara is a darker form of brown sugar. Most brown sugar is really just white sugar with molasses added back in to it to darken it up, so you could probably get away with regular brown sugar. But are you sure you want to make beer with brown sugar in it?

Demarara sugar is the light brown, non sticky stuff you get given with a cappuccino,and as previously stated it is normal sugar that has a small amount of molasses added.Your kneck of the woods may be different but at your local supermarket you should be able to find white sugar( hard white stuff),light brown( same texture but light brown in colour)- this is Demarara, brown(getting sticky) and dark brown(sticky). Personally,i wouldnt give too much importance to the type of sugar used if used at all that is, if doing an all grain recipe, just notch up the mash by a degree or two or lose the odd liquor to grain ratio in favour of the grain. All they are doing in the recipe is adding a fermentable with a touch of dextrinas quality. The books by Dave Line in the 70's were a big exponent of different sugar types.I found that they can all be modified with a little mash temp tweeking here and there to be able to leave the sugar out completely